Why does blindness heighten other senses?

We’ve all heard about the amazing ability of some blind people to hear, smell, or touch with such a high degree of acuity that they become almost savant-like. Daniel Kish, for example, has become famous for his ability to use echolocation, like bats or dolphins, to navigate the world.

But Kish was blinded at the tender age of 13 months, and many other blind people with hyper-ability in other senses were born that way. What about the rest of us? Do we have innate abilities we almost never use?

At this week’s Acoustics 2012 scientific meeting in Hong Kong, a team of Canadian researchers from the auditory neuroscience lab of François Champoux at the University of Montreal presented a study they conducted with sighted people. According to Simon Landry, a graduate student, the researchers exposed subjects to a harmonic tone. Such tones sound like a single note, but they actually have layers of “harmonicity.” So the team slightly altered one layer until the subjects could notice it.

All sighted subjects were about the same in their ability to distinguish an altered layer. But in a second round of testing, those who spent just 90 minutes blindfolded performed significantly better than non-blindfolded participants, and better than they themselves did the first time around.

How the brain can do this hasn’t yet been fully established, but, Landry explained, “the idea is that the brain doesn’t actually change, but vision no longer suppresses the processing of other modalities, which have existing pathways, in the visual cortex.”

By “modalities,” Landry means types of sensory input. He’s using the language of Alvaro Pascual-Leone, a Harvard behavioral neurologist who has spent years studying how the brain processes information.

Pascual-Leone refers to the brain as “metamodal.” He sees the results of the Montreal researchers as “amazingly remarkable…. This is one more illustration” of the metamodal hypothesis “and the implication to me is that clearly there is a cross-talk betweens the senses that goes well beyond what we thought. We have been thinking of these systems as silos, independent of each other, and that is definitely not the case.”

In Pascual-Leone’s hypothesis, parts of the brain aren’t firmly predestined to translate vision or touch or sound, they are simply biased toward one or another by the way they develop. Then, when we open our eyes as newborns, the visual information tends to be translated by the occipital cortex because it’s best suited for the job, not because it’s the only region that can do it, nor because that’s all it can do. All that visual information streaming in just overwhelms information from our ears or our fingers. In his view, the sensory pats of our brains aren’t silos, but a web.

“Why do you close your eyes when you go to a concert?” he asks. “You are suppressing the visual input. That disinhibits the connection between the visual and auditory cortices” so information can flow between them. “It makes more of the brain able to process sound.”

Take, for example, the case of a Spanish woman who was blind since birth. She worked as a Braille proof reader for the Spanish Organization for the Blind. At age 62, she suffered a stroke that resulted in a coma from which she recovered. But she was no longer able to read Braille. Her stroke had injured her occipital cortex, the center of vision. Her sense of touch was unimpaired. Though she had ever been able to see, her occipital cortex was instrumental in making sense of what she touched and turning it into language.

Pascual-Leone pointed out that the Canadian experiment underscores the difference between those blind from birth and those who become blind later in life. Those who have experienced vision “have a brain that’s been calibrated with vision.” They have, metaphorically speaking, acquired a library of references that have physically reshaped their brains. A congenitally blind person knows what, for example, a cube and a sphere feel like. But if they gain sight later in life, and see a cube and sphere together for the first time, they can’t tell which is which “unless they can match the visual with the tactile,” he said. After that integration, they do fine.

“What we find with sighted, blindfolded people,” he said, “may be showing us something about what it is to see, rather than to be blind.”